1. Field of the Invention
The present invention consists in a method and a machine for welding thermoformed sheets of plastics material.
2. Description of the Related Art
Welding thermoformed plastics material sheets together to obtain cellular blocks for many applications is known in itself. For example, profiled thermoformed PVC (Polyvinyl Chloride) sheets have until now been assembled by ultrasonic welding, by high-frequency welding or by gluing.
An assembly method of the above kind is reliable for PVC but when this material burns it releases chlorine which combines with hydrogen and oxygen in the air to form the toxic gas hypochlorite.
It has therefore been proposed to replace the PVC with other, less harmful materials, ones not containing chlorine, and the combustion of which produces only water. Polypropylene and polyethylene are advantageously used, for example. Unfortunately, ultrasonic welding of thermoformed sheets of either of these materials is in practice difficult, time-consuming and costly. Moreover, high-frequency welding is impossible with these materials. Gluing is possible, on the other hand, using a glue suitable for polypropylene, for example Araldite.RTM., as an adhesive. However, this particular adhesive ages badly, absorbs water and in the final analysis is not compatible with the more frequent applications of the thermoformed sheets, in particular when they are used to make cellular blocks for embankments, underground storage tanks, etc.
The conventional welding methods are summarized below:
Clamp welding: This method consists in clamping the two elements to be welded between two fingers heated to a sufficient temperature to melt the plastics material. The heating is effected by contact which means that the molten material can adhere to the finger when it is withdrawn, which is a significant drawback of this method. Also, it is not possible to weld more than two elements at a time, unless the lower finger is dispensed with. PA1 Mirror welding: This method is widely used in the field of plastics material, in particular for butt jointing tubes. The two tubes to be welded are placed end-to-end on a support with a gap of a few millimeters between their ends. This method has the advantage of simplicity, but it is more particularly suited to elements of large section. When it is withdrawn, the mirror removes a small amount of molten material from the elements to be welded. In the case of butt welding plastic tube this is of no consequence, but for a sheet a few hundreds of microns thick the damage caused can be significant. PA1 Ultrasound welding: This method has the advantage of enabling welding in places where access is difficult for other, more conventional methods. However, it requires an ultrasound generator, which is often costly, and it is often impossible to weld more than two elements at the same time because to avoid dissipating the vibrating energy there cannot be a plurality of stacked welding levels between the anvil and the "sonotrode" (the device generating the ultrasounds). Also, the minimum pressure to be applied by the sonotrode is relatively high, which means that this method finds more particular application to rigid structures or plane elements. Finally, the efficiency of transmission of vibratory energy between the sonotrode and the contact surface of the elements to be welded is in inverse proportion to their modulus of elasticity. PA1 Infra-red welding: The heat source is a battery of infra-red lamps which heat one or both of the elements to be welded, causing them to melt at certain points. It is then sufficient to apply a force between the two elements in the molten areas to weld them. This method has definite advantages, but also entails the difficulty of channeling the thermal radiation towards the welding surfaces. The area adjoining the contact surface can absorb some of the incident radiation and this increase in temperature can soften it, even partially melt it, and deform it. It would then be impossible to apply a force to the elements without damaging them, which would be particularly harmful in the case of thin thermoformed sheets.